Patent application title: PROCESS FOR THE PRODUCTION OF DIMETHYL ETHER

Abstract:

A process for the production of dimethyl ether from a methanol reactor
effluent is disclosed. The process may include: contacting an aqueous
extractant comprising water and an effluent from a methanol synthesis
reactor comprising methanol and one or more of methane, water, carbon
monoxide, carbon dioxide, hydrogen, and nitrogen. At least a portion of
the methanol partitions into the aqueous extractant; recovering an
extract fraction comprising the aqueous extractant and methanol. The
extract fraction is fed to a catalytic distillation reactor system for
concurrently: contacting the methanol with catalyst in a reaction zone
thereby catalytically reacting at least a portion of the methanol to form
dimethyl ether and water; and fractionating the resulting dimethyl ether
and the water to recover a first overheads fraction comprising dimethyl
ether and a first bottoms fraction comprising water.

Claims:

1. A process for the production of dimethyl ether, the process
comprising:contacting an aqueous extractant comprising water and an
effluent from a methanol synthesis reactor, in a partial or total vapor
phase and comprising methanol and one or more of methane, water, carbon
monoxide, carbon dioxide, hydrogen, and nitrogen, whereby at least a
portion of the methanol partitions into the aqueous extractant;recovering
an extract fraction comprising the aqueous extractant and
methanol;recovering a raffinate fraction comprising the one or more of
methane, water, carbon monoxide, carbon dioxide, hydrogen, and
nitrogen;feeding the extract fraction to a catalytic distillation reactor
system;concurrently in the catalytic distillation reactor system;i)
contacting the methanol with a catalyst in a distillation reaction zone
thereby catalytically reacting at least a portion of the methanol to form
dimethyl ether and water; andii) fractionating the resulting dimethyl
ether and the water to recover a first overheads fraction comprising
dimethyl ether and a first bottoms fraction comprising water.

2. The process of claim 1,wherein the extract fraction further comprises
at least one of nitrogen, carbon monoxide and carbon dioxide; andwherein
the overheads fraction further comprises at least one of C2 to C4
olefins, carbon monoxide, nitrogen, and carbon dioxide;the process
further comprising:separating the first overheads fraction via fractional
distillation to recover a second overheads fraction comprising the at
least one of nitrogen, carbon monoxide, carbon dioxide and unreacted
methanol and a second bottoms fraction comprising dimethyl ether.

3. The process of claim 2, wherein the second overheads fraction comprises
at least about 99.5 weight percent dimethyl ether.

4. The process of claim 3, wherein the first overheads fraction comprises
at least about 99.8 weight percent dimethyl ether.

5. The process of claim 2, further comprising recycling at least a portion
of the second overheads fraction to the methanol synthesis reactor.

6. The process of claim 1, wherein the first bottoms comprises at least
about 90 weight percent water.

7. The process of claim 6, further comprising recycling at least a portion
of the first bottoms as the aqueous extractant.

8. The process of claim 1, wherein the catalyst in the distillation
reaction zone comprises at least one of a metalized resin catalyst, a
silica-alumina catalyst, and mixtures thereof.

9. The process of claim 1, further comprising:contacting the extract
fraction with a catalyst in a fixed bed reaction zone thereby
catalytically reacting at least a portion of the methanol to form an
effluent comprising methanol, water, and dimethyl ether;feeding the
effluent to the catalytic distillation reactor system as the extract
fraction.

10. The process of claim 9,wherein the catalyst in the fixed bed reaction
zone comprises at least one of a metalized resin catalyst, a
silica-alumina catalyst, and mixtures thereof; andwherein the catalyst in
the distillation reaction zone comprises at least one of a metalized
resin catalyst, a silica-alumina catalyst, and mixtures thereof

11. The process of claim 1, further comprising recycling the raffinate
fraction to the methanol reactor.

12. The process of claim 11, further comprising contacting the raffinate
fraction in indirect heat exchange with the effluent from the methanol
reactor.

13. The process of claim 1, wherein the contacting an aqueous extractant
is conducted at a temperature in the range of 200.degree. F. to about
500.degree. F. and a pressure in the range from about 500 psig to about
2000 psig.

14. The process of claim 13, wherein the pressure during the contacting is
within about 15% of the operating pressure of the methanol reactor.

15. The process of claim 1, wherein the distillation reaction zone is at a
pressure in the range from about 200 to about 500 psig and a temperature
in the range from about 50.degree. F. to about 500.degree. F.

16. The process of claim 1, wherein the contacting an aqueous extractant
is conducted in at least one of an absorber column and an extractive
distillation column.

17. A process for the production of dimethyl ether, the process
comprising:contacting an aqueous extractant comprising water and an
effluent from a methanol synthesis reactor, in a partial or total vapor
phase and comprising methanol and one or more of methane, water, carbon
monoxide, carbon dioxide, hydrogen, and nitrogen, whereby at least a
portion of the methanol partitions into the aqueous extractant;recovering
an extract fraction comprising the aqueous extractant and
methanol;recovering a raffinate fraction comprising the one or more of
methane, water, carbon monoxide, carbon dioxide, hydrogen, and
nitrogen;contacting the raffinate fraction in indirect heat exchange with
the effluent from the methanol reactor;recycling the heat exchanged
raffinate fraction to the methanol reactor;feeding the extract fraction
to a catalytic distillation reactor system;concurrently in the catalytic
distillation reactor system;i) contacting the methanol with a catalyst in
a distillation reaction zone thereby catalytically reacting at least a
portion of the methanol to form dimethyl ether and water; andii)
fractionating the resulting dimethyl ether and the water to recover a
first overheads fraction comprising dimethyl ether and at least one of C2
to C4 olefins, carbon monoxide, nitrogen, and carbon dioxide and a first
bottoms fraction comprising water;separating the first overheads fraction
via fractional distillation to recover a second overheads fraction
comprising the at least one of nitrogen, carbon monoxide, carbon dioxide
and C2 to C4 olefins and a second bottoms fraction comprising dimethyl
ether.

18. The process of claim 17, further comprising recycling at least a
portion of the second overheads fraction to at least one of the methanol
synthesis reactor and a syngas reactor.

19. The process of claim 17, further comprising recycling at least a
portion of the first bottoms fraction to the methanol recovery system as
the aqueous extractant.

20. The process of claim 17, further comprisingcontacting the extract
fraction with a catalyst in a fixed bed reaction zone thereby
catalytically reacting at least a portion of the methanol to form an
effluent comprising methanol, water, and dimethyl ether;feeding the
effluent to the catalytic distillation reactor system as the extract
fraction.

Description:

FIELD OF THE DISCLOSURE

[0001]Embodiments disclosed herein relate to processes for the production
of dimethyl ether (DME) from methanol. More particularly, embodiments
disclosed herein relate to processes for separating methanol from a feed
gas, such as an effluent from a methanol synthesis reactor, where the
methanol is reacted for production of dimethyl ether. The separation of
the methanol from the feed gas is preferably conducted at conditions
sufficient to minimize the heating and compression requirements for
recycling of gaseous product streams to the methanol synthesis reactor.

BACKGROUND

[0002]DME is a commercially valuable product. For example, DME serves as a
building block for the production of numerous chemicals. DME may be used,
for example, as a component of chemical reactions, as an additive in
liquefied petroleum gas, and also as a clean-burning or diesel
replacement fuel.

[0003]Methanol, as a raw material, may be produced from natural gas. DME
may thus be produced from methane by first converting methane in natural
gas into methanol. Natural gas typically contains about 60 to 100 mole
percent methane, the balance being primarily heavier alkanes. Alkanes of
increasing carbon number are normally present in decreasing amounts.
Carbon dioxide, hydrogen sulfide, nitrogen, and other gases may also be
present in relatively low concentrations. Natural gas is a common and
economical feedstock for producing methanol, although other feedstocks
may also be used.

[0004]A typical methanol synthesis reactor (for conversion of syngas to
methanol) will convert only about 20% to 60% of the syngas fed to the
reactor in a single pass. To obtain higher conversions, the unreacted
syngas is typically separated from the product methanol and recycled back
to the reactor or directed to a second reactor to produce additional
methanol. Methanol synthesis reactors are disclosed in, for example, U.S.
Pat. Nos. 4,968,722, 5,219,891, 5,449,696, 6,723,886, and 5,177,114 and
GB 2092172A, each of which are incorporated herein by reference to the
extent they are not contradictory to embodiments disclosed herein.

[0005]Methanol synthesis reactors are typically operated at relatively
high temperatures and pressures, for example, from about 400° F.
to about 600° F. and from about 1000 psig to about 1500 psig. The
requirement of a high temperature and pressure adds costs to the process
in terms of energy and capital expenditures. Savings on energy costs and
capital costs associated with pre-heating and pressurizing the feed gases
to the methanol reactor would be beneficial to the process. Due to the
low conversion per pass and high recycle requirement, a significant cost
is associated with compression and heating of recycle gases following
separation of the methanol product from unreacted gases in the methanol
synthesis reactor effluent.

[0006]Accordingly, there exists a need for a process for the production of
dimethyl ethers from methanol synthesis reactor effluents that provides
energy savings and greater efficiency over conventional processes.

SUMMARY OF CLAIMED EMBODIMENTS

[0007]In one aspect, embodiments disclosed herein relate to a process for
the production of dimethyl ether. The process may include: contacting an
aqueous extractant comprising water and an effluent from a methanol
synthesis reactor, in a partial or total vapor phase and comprising
methanol and one or more of methane, water, carbon monoxide, carbon
dioxide, hydrogen, and nitrogen, whereby at least a portion of the
methanol partitions into the aqueous extractant; recovering an extract
fraction comprising the aqueous extractant and methanol; recovering a
raffinate fraction comprising the one or more of methane, water, carbon
monoxide, carbon dioxide, hydrogen, and nitrogen; feeding the extract
fraction to a catalytic distillation reactor system; concurrently in the
catalytic distillation reactor system; contacting the methanol with a
catalyst in a distillation reaction zone thereby catalytically reacting
at least a portion of the methanol to form dimethyl ether and water; and
fractionating the resulting dimethyl ether and the water to recover a
first overheads fraction comprising dimethyl ether and a first bottoms
fraction comprising water.

[0008]In another aspect, embodiments disclosed herein relate to a process
for the production of dimethyl ether. The process may include: contacting
an aqueous extractant comprising water and an effluent from a methanol
synthesis reactor, in a partial or total vapor phase and comprising
methanol and one or more of methane, water, carbon monoxide, carbon
dioxide, hydrogen, and nitrogen, whereby at least a portion of the
methanol partitions into the aqueous extractant; recovering an extract
fraction comprising the aqueous extractant and methanol; recovering a
raffinate fraction comprising the one or more of methane, water, carbon
monoxide, carbon dioxide, hydrogen, and nitrogen; contacting the
raffinate fraction in indirect heat exchange with the effluent from the
methanol reactor; recycling the heat exchanged raffinate fraction to the
methanol reactor; feeding the extract fraction to a catalytic
distillation reactor system; concurrently in the catalytic distillation
reactor system; contacting the methanol with a catalyst in a distillation
reaction zone thereby catalytically reacting at least a portion of the
methanol to form dimethyl ether and water; and fractionating the
resulting dimethyl ether and the water to recover a first overheads
fraction comprising dimethyl ether and at least one of C2 to C4 olefins,
carbon monoxide, nitrogen, and carbon dioxide and a first bottoms
fraction comprising water; separating the first overheads fraction via
fractional distillation to recover a second overheads fraction comprising
the at least one of nitrogen, carbon monoxide, carbon dioxide and C2 to
C4 olefins and a second bottoms fraction comprising dimethyl ether.

[0009]Other aspects and advantages of the invention will be apparent from
the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0010]FIG. 1 is a simplified process flow diagram according to embodiments
disclosed herein.

[0011]FIG. 2 is a simplified process flow diagram according to embodiments
disclosed herein.

DETAILED DESCRIPTION

[0012]Within the scope of this application, the expression "catalytic
distillation reactor system" denotes an apparatus in which the alcohol
condensation reaction and the separation of products take place at least
partially simultaneously. The apparatus may include a conventional
catalytic distillation column reactor, where the reaction and
distillation are concurrently taking place at boiling point conditions,
or a distillation column combined with at least one side reactor, where
the side reactor may be operated as a liquid phase reactor or a boiling
point reactor, or a combination of these. While both catalytic
distillation processes may be preferred over conventional liquid phase
reaction followed by separations, a catalytic distillation column reactor
may have the advantages of decreased piece count, efficient heat removal
(heat of reaction may be absorbed into the heat of vaporization of the
mixture), and a potential for shifting equilibrium.

[0013]In one aspect, embodiments disclosed herein relate to processes for
the production of dimethyl ethers. More specifically, embodiments
disclosed herein relate to processes for the production of dimethyl ether
(DME) from methanol. More particularly, embodiments disclosed herein
relate to processes for separating methanol from a feed gas, such as an
effluent from a methanol synthesis reactor, where the methanol recovered
is subsequently reacted to produce dimethyl ether.

[0014]Feedstocks to processes for the production of dimethyl ether
according to embodiments disclosed herein may include effluent streams
from methanol synthesis reactors. As used herein, methanol synthesis
reactors are defined as reactors for producing methanol from a synthesis
gas, a pyrolysis gas, or other streams containing hydrogen, carbon
monoxide, and carbon dioxide. Methanol synthesis reactor effluents may
include methanol, as well as unreacted gases including hydrogen, methane,
carbon monoxide, carbon dioxide, and nitrogen, among others. The
feedstock may exit these processes at relatively high temperatures and
pressures, such as temperatures in the range from about 400° F. to
600° F. and pressures in the range from about 500 psig to about
2000 psig.

[0015]The methanol synthesis reactor effluent, which may be in a partial
or total vapor phase, may be fed to a methanol recovery system where the
effluent is contacted system with an aqueous extractant, including at
least one of water, methanol, and dimethyl ether, to separate at least a
portion of the methanol from the unreacted gases present. Such contacting
may be conducted, for example, in an extractive distillation column, an
absorber column, or other equipment known to those skilled in the art for
partitioning a component from a vapor phase into a liquid phase. An
extract fraction, including the aqueous extractant and methanol, and a
raffinate fraction, including the unreacted gases and any remaining
methanol, may each be recovered for further processing.

[0016]The raffinate fraction may be recycled to the methanol synthesis
reactor or a syngas reactor upstream of a methanol synthesis reactor for
production of additional methanol. Alternatively, the raffinate fraction
may be directed to a secondary methanol synthesis reactor. To increase
the pressure and/or temperature of the raffinate fraction for feed to the
methanol synthesis reactor, heat exchange and/or compression may be
required. In some embodiments, the raffinate fraction may be contacted in
indirect heat exchange with the methanol synthesis reactor effluent to
increase a temperature of the raffinate fraction for feeding at elevated
temperatures to the methanol synthesis reactor.

[0017]Operating conditions in the methanol recovery system may include a
temperature in the range from about 200° F. to about 500°
F., pressures in a range from about 500 psig to about 2000 psig, a gas to
aqueous extractant mole ratio from about 2 to about 10, such as about 4
to about 6. In some embodiments, operating pressures in the methanol
recovery system may be within about 20% of the operating pressure of the
methanol synthesis reactor; within about 15% in other embodiments; within
about 10% in other embodiments; and within about 5% in other embodiments.
Operating the methanol recovery system at pressures comparable to the
operating pressure of the methanol synthesis reactor results in a
raffinate fraction having a similar pressure to that for feeding of the
reactants to themethanol synthesis reactor, thus reducing compression
requirements. Recycle of the unreacted gases may improve upon the overall
efficiency of the overall process, converting additional methane to
methanol, reducing raw material costs and improving the overall
conversion of the process to methanol and/or dimethyl ether.

[0018]The extract fraction may then be fed to a reaction system for the
conversion of methanol to dimethyl ether, which may include a catalytic
distillation reactor system, or a combination of a fixed bed reactor and
a catalytic distillation reactor system.

[0019]Concurrently in the catalytic distillation reactor system, i) the
methanol is contacted with a catalyst in a distillation reaction zone
thereby catalytically reacting at least a portion of the methanol to form
dimethyl ether and water; and ii) the resulting dimethyl ether, product
water, and aqueous extractant are separated to recover an overheads
fraction including dimethyl ether and a bottoms fraction including water.

[0020]Use of a catalytic distillation reactor system for the conversion of
methanol to dimethyl ether is advantageous as the catalytic distillation
reactor system combines the reaction to produce dimethyl ether with the
separation of the product water as a separate stream. Water is useful as
a selective absorbent for methanol, as noted above, and the catalytic
distillation reactor system may thus process the absorbent water and the
product water simultaneously.

[0021]The catalytic distillation reactor system may include one or more
reaction zones containing a catalyst for promoting the conversion of
methanol to dimethyl ether, where the reaction zones may be located in
the rectification zone and/or the stripping zone of the catalytic
distillation reactor system.

[0022]In other embodiments, a fixed bed reactor may be used upstream of
the catalytic distillation reactor system. The fixed bed reactor may
convert at least a portion of the methanol to dimethyl ether, and the
effluent from the fixed bed reactor may then be fed to the catalytic
distillation reactor system for additional conversion of methanol to
dimethyl ether and concurrent separation of the dimethyl ether from the
water (present as a reaction product and as the aqueous extractant that
may be fed to the distillation column reactor system).

[0023]The fixed bed reactor may be operated liquid continuous, or may be
operated at a boiling point of the reaction mixture, such as in a down
flow boiling point reactor or a pulse flow reactor. Operating conditions
in the fixed bed reactor may be selected to achieve partial conversion of
methanol, such as at least 25 weight percent of the methanol; or at least
50 weight percent in other embodiments.

[0024]In other embodiments, operating conditions in the fixed bed reactor
may be selected to achieve reaction equilibrium. For example, methanol
dehydration to dimethyl ether may have a thermodynamic equilibrium
limitation of approximately 80-87 weight percent conversion of the
alcohol. The resulting mixture may then be fed to the catalyst
distillation reactor system for additional conversion, as greater than
equilibrium conversion may be attained in a catalytic distillation
reactor system due to the continuous removal of products from the
reaction zone. In some embodiments, due to the concurrent fractionation
and separation of reactants and products, essentially complete conversion
of the methanol may be obtained in the distillation column reactor
system.

[0025]Operating conditions in the fixed bed reactor may include a
temperature in the range from about 50° F. to about 500°
F., and pressures in a range from about 5 psig to about 750 psig.

[0026]Operating conditions in the distillation column reactor system may
include a temperature in the range from about 50° F. to about
500° F., pressures in a range from about 200 psig to about 500
psig, such as in the range from about 250 psig to about 350 psig, and a
reflux ratio (L/D) from about 2 to about 10, such as about 3 to about 5.

[0027]The dimethyl ether may be recovered as an overheads fraction, which
may be essentially pure dimethyl ether in some embodiments. Water, formed
during the condensation reaction, may be recovered as a bottoms fraction,
which may be essentially pure water in some embodiments. Essentially
pure, as used herein, refers to a composition or mixture, such as the
bottoms fraction or overheads fraction, containing at least 95 weight
percent of the indicated compound, such as the dimethyl ether or the
water. In other embodiments, the recovered fractions may contain at least
98 weight percent of the indicated compound; at least 98.5 weight percent
of the indicated compound; at least 99 weight percent in other
embodiments; at least 99.5 weight percent in other embodiments; at least
99.8 weight percent in other embodiments; and at least 99.9 weight
percent in yet other embodiments.

[0028]In some embodiments, the bottoms fraction may contain less than 5
weight percent methanol. In other embodiments, the bottoms fraction may
contain less than 1 wt % methanol; less than 5000 ppm by weight methanol
in other embodiments; less than 1000 ppm by weight methanol in other
embodiments; and less than 500 ppm by weight methanol in yet other
embodiments.

[0029]Side reaction products may include light hydrocarbons, such as C2 to
C4 olefins, as well as heavier components, such as oligomeric or
polymeric compounds. The higher boiling materials may foul the catalyst,
or may be washed down the column and exit with the bottoms fraction.
Light components formed, such as light olefins (C2 to C4 olefins) may
exit the distillation column reactor system with the overheads fraction.
Additionally, carbon dioxide, carbon monoxide, and nitrogen may be
entrained or dissolved in the aqueous extractant, fed to the distillation
column reactor system, and recovered with the overheads fraction. These
are each typically minority components and do not significantly affect
the purity of the product streams.

[0030]Although embodiments of processes disclosed herein may result in the
production of substantially pure dimethyl ether and water product
streams, these streams may also undergo subsequent treatment. The need
for subsequent treatment may depend upon the quality of the alcohol feed,
the amount and type of reaction byproducts, as well as the amount and
type of entrained or dissolved gases in the extract fraction recovered
from the methanol recovery system. Subsequent treatment of the product
streams may include, for example, treatment of the dimethyl ether stream
with an acidic ion exchanger to remove odor-producing impurities. Other
treatments may include the removal of heavier organic reaction byproducts
from the water stream.

[0031]In some embodiments, the extract fraction may include at least one
of nitrogen, carbon monoxide and carbon dioxide, and the overheads
fraction may include at least one of carbon monoxide, nitrogen, carbon
dioxide, and C2 to C4 olefins. The overheads fraction may then be
separated via fractional distillation to recover an overheads fraction
including the at least one of nitrogen, carbon monoxide, carbon dioxide
and C2 to C4 olefins and a bottoms fraction including the dimethyl ether,
which may be a high purity dimethyl ether stream. In some embodiments,
the bottoms fraction from the fractional distillation may contain less
than 5 mole % carbon dioxide; less than 1 mole % carbon dioxide in other
embodiments; less than 5000 mole ppm in other embodiments; less than 1000
mole ppm in other embodiments; and less than 500 mole ppm in yet other
embodiments.

[0032]Referring now to FIG. 1, a simplified process flow diagram for the
production of dimethyl ethers according to embodiments disclosed herein
is illustrated. One skilled in the art would recognize that, although not
depicted, pumps, valves, vessels, storage tanks, and other equipment
commonly used for the processes described and illustrated herein are not
shown so as to simplify the diagram.

[0033]A feed stream from a methanol synthesis reactor, containing methanol
and other gases, is fed via fluid conduit 2 to methanol recovery system
6. Aqueous extactant, such as water, is fed via fluid conduit 4 to
methanol recovery system 6. In methanol recovery system 6, the aqueous
extractant and feed stream are contacted to absorb at least a portion of
the methanol in the aqueous extractant. Concurrently, the resulting
absorbed methanol, water, and other gases are separated to recover a
raffinate fraction, containing the non-absorbed gases and recovered via
flow conduit 8, and a extract fraction, containing the absorbed methanol
and recovered via flow conduit 10. At least a portion of the non-absorbed
gases in the raffinate fraction may be recycled back to a methanol
synthesis reactor (not shown) for additional conversion of the gaseous
components to methanol.

[0034]The extract fraction is then fed via flow line 10 to a catalytic
distillation reactor system 12. In catalytic distillation reactor system
12, the absorbed methanol is contacted with a catalytic distillation
structure in a distillation reaction zone 14 to catalytically react a
portion of the absorbed methanol to form corresponding dimethyl ethers
and water. While the reaction is proceeding, the reaction products are
concurrently fractionated, allowing dimethyl ether to be recovered as a
first overheads fraction via flow line 16 and water to be recovered as a
first bottoms fraction via flow line 18.

[0035]If necessary, the second overheads may be fed via flow line 16 to a
fractional distillation column 20 to further purify the dimethyl ether,
recovering a second overheads fraction 22 that includes light gases, such
as entrained or dissolved carbon dioxide from the methanol recovery
system 6 or light hydrocarbons produced in catalytic distillation reactor
system 12, and recovering a second bottoms 24 that includes dimethyl
ether.

[0036]Referring now to FIG. 2, a simplified process flow diagram of a
process for the production of dimethyl ethers according to other
embodiments disclosed herein is illustrated, where like numerals
represent like parts. In this embodiment, the extract fraction, including
methanol and the aqueous extractant, such as water, may be fed to a fixed
bed reactor 30 for conversion of at least a portion of the methanol to
dimethyl ether prior to feed of the extract fraction to distillation
column reactor system 12.

[0037]Additionally illustrated in the embodiment of FIG. 2, at least a
portion of the bottoms fraction recovered via flow line 18 may be
recycled as the extractant fraction 4. The raffinate fraction recovered
via flow line 8 is contacted in indirect heat exchange with the effluent
in flow line 2 from methanol synthesis reactor 32 in heat exchanger 34.
The second overheads fraction recovered via flow line 22 may be
compressed via compressor 36 and heated via indirect heat exchange via
heat exchanger 38. The heated raffinate fraction and the compressed and
heated second overheads fraction may then be recycled via flow lines 40
and 42, respectively, to an inlet of methanol synthesis reactor 32 along
with fresh synthesis gas fed via flow line 44. Various purge streams,
heat exchangers, pumps, compressors, and other equipment may also be used
to properly integrate methanol synthesis reactor 32 with methanol
recovery system 6 and dimethyl ether recovery system 20.

[0038]Catalysts that may be used in the fixed bed reactor and the
distillation column reactor system are dehydration catalysts, usually
characterized as acidic dehydration catalysts. Zeolites and metal
substituted cationic resin catalysts may be used for this reaction, but
other mildly acidic catalyst may also be used.

[0039]Naturally occurring zeolites have irregular pore size and are not
generally considered as equivalent to synthetic zeolites. In some
embodiments, however, naturally occurring zeolites are acceptable so long
as they are substantially pure. The balance of the present discussion
shall be directed to the synthetic zeolites with the understanding that
natural zeolites are considered equivalent thereto as indicated above,
i.e., in so far as the natural zeolites are the functional equivalents to
the synthetic zeolites.

[0040]Synthetic zeolites may be prepared in the sodium form, that is, with
a sodium cation in close proximity to each aluminum tetrahedron and
balancing its charge. A number of principal types of molecular sieves
have been reported, such as A, X, Y, L, erionite, omega, beta, and
mordenite. The A-type molecular sieves have relatively small pore size.
By the term pore size is meant the effective pore size (diameter) rather
than the free pore size (diameter). X- and Y-type molecular sieves
generally have a larger pore size (approximately 7.4 Å) and differ as
to the range of ratio of Al2O3 to SiO2. Type L and other
types listed have still higher ratios of SiO, to Al2O3, as
known in the art.

[0041]Zeolite catalysts that may be used in embodiments disclosed herein
are the acid form of the zeolite or at least exhibit acidic
characteristics. The acid form is commercially available, but also may be
prepared by treating the zeolites with acid to exchange Na for hydrogen.
Another method to produce the acid form is to treat the zeolite with
decomposable cations (generally ammonium ions) to replace Na with the
decomposable ions and thereafter to heat the mole sieve to decompose the
cation leaving the acid form. Generally the Na form is treated with
ammonium hydroxide to remove the Na and thereafter the zeolite is heated
to a temperature of about 350° C. to remove the ammonia. The
removal of Na.sup.+ ions with NH4.sup.+ is more easily carried out
than with multivalent ions, as described below, and these catalysts are
generally more active, but less stable to heat than the multivalent
cation exchange forms. Zeolites, which have had their alkali metal
reduced to low levels by partial treatment with NH4.sup.+ and
partial multivalent metal cation exchange, may be expected to possess
increased activity and increased stability.

[0042]Pore size within the crystal lattice may be significant in this
reaction. According to one theory of molecular sieve catalytic activity,
zeolite catalysis occurs primarily inside the uniform crystal cavities;
consequently, zeolitic catalyst activity depends on the number of
aluminum atoms in the crystal and thus on the chemical composition of the
crystal. Moreover, these catalytic sites are fixed within the rigid
structure of the crystal, meaning that access to active sites can be
altered by altering the structure of the crystal.

[0043]In some embodiments, resin catalysts may be used. For example, resin
catalyst compositions such as sulfonic acid resins which have at least
50% of the sulfonic acid groups neutralized with one or more metal ions
of Groups 4-12 of the Periodic Table, the rare earth metals, or mixtures
thereof. The balance of the sulfonic acid groups may be neutralized with
an alkali metal or alkaline earth metal, ammonium, or mixtures thereof.
The sulfonic acid may be attached to any polymeric backbone. In some
embodiments, the metal ions may include one or more of Ti, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Ta, W, Re, Pt, Ce, Nd,
Sm, and Eu. The metal modified resin catalyst compositions are disclosed
in U.S. Pat. Nos. 4,551,567 and 4,629,710, each of which are incorporated
herein.

[0044]Acid cation exchange resins are well known and have a wide variety
of uses. The resins are cation exchangers that contain sulfonic acid
groups which may be obtained by polymerization or copolymerization of
aromatic vinyl compounds followed by sulfonation. Aromatic vinyl
compounds suitable for preparing polymers or copolymers are: styrene,
vinyl toluene, vinyl naphthalene, vinyl ethylbenzene, methyl styrene,
vinyl chlorobenzene, and vinyl xylene. A large variety of methods may be
used for preparing these polymers. For example, polymerization alone or
in admixture with other monovinyl compounds, or by crosslinking with
polyvinyl compounds, such as divinyl benzene, divinyl toluene, and
divinylphenylether, among others. The polymers may be prepared in the
presence or absence of solvents or dispersing agents, and various
polymerization initiators may be used, e.g., inorganic or organic
peroxides, persulfates, etc.

[0045]The sulfonic acid group may be introduced into these vinyl aromatic
polymers by various known methods; for example, by sulfating the polymers
with concentrated sulfuric and chlorosulfonic acid, or by copolymerizing
aromatic compounds which contain sulfonic acid groups (see e.g., U.S.
Pat. No. 2,366,007). Further sulfonic acid groups may be introduced into
the polymers which already contain sulfonic acid groups; for example, by
treatment with fuming sulfuric acid, i.e., sulfuric acid which contains
sulfur trioxide. The treatment with fuming sulfuric acid is preferably
carried out at 0 to 150° C. and the sulfuric acid should contain
sufficient sulfur trioxide so that it still contains 10 to 50% free
sulfur trioxide after the reaction. The resulting products may contain an
average of 1.3 to 1.8 sulfonic acid groups per aromatic nucleus.
Particularly, suitable polymers containing sulfonic acid groups are
copolymers of aromatic monovinyl compounds with aromatic polyvinyl
compounds, particularly, divinyl compounds, in which the polyvinyl
benzene content is preferably 1 to 20% by weight of the copolymer (see,
for example, DE 908,247).

[0046]The ion exchange resin may have a granular size of about 0.25 to 1
mm, although particles from 0.15 mm up to about 2 mm may be used. The
finer catalysts provide high surface area, but also result in high
pressure drops through the reactor. The macroreticular form of these
catalysts have a much larger surface area exposed and undergo limited
swelling in a non-aqueous hydrocarbon medium compared to the gelular
catalysts.

[0047]The metal modified catalyst may be prepared by contacting a
macroporous matrix containing a sulfonic acid group with an aqueous
solution of metal salts and solutions of alkali metal salts, alkaline
earth metal salts, and/or ammonium salts to neutralize the acid groups.
An alternative procedure for the preparation of the metal modified cation
resin catalyst compositions comprises contacting a sulfonic acid cation
exchange resin, e.g., a macroporous matrix of a polyvinyl aromatic
compound crosslinked with a divinyl compound and having thereon from
about 3 to 5 milli-equivalents of sulfonic acid groups per gram of dry
resin, (1) with an aqueous solution of a soluble metal salt as described
above, such as Al, Fe, Zn, Cu, Ni, or mixtures thereof, to neutralize at
least 50% to less than 100% of the available sulfonic acid groups with
metal ions to produce a partially neutralized resin, and (2) thereafter
contacting the partially neutralized resin with an aqueous solution
containing a soluble compound of an alkali or alkaline earth metal of
Groups 1 or 2, of the Periodic Table, or mixture thereof to neutralize
the remaining sulfonic acid groups. In the final alkali neutralization
step under the alternate procedure, care must be exercised to not contact
the partially neutralized resin with a large excess of alkali or alkaline
earth metal ions, (a slight excess, up to about 20%, beyond that required
to neutralize the residual sulfonic acid groups may be used) since they
appear to form double salts or possibly elute the metal ions, which may
reduce the activity of the catalyst.

[0048]Resin catalyst composition useful herein may be characterized as a
solid comprising a macroporous matrix of polyvinyl aromatic compound
crosslinked with a divinyl compound and having thereon from about 3 to 5
milli-equivalents of sulfonic acid groups per gram of dry resin, wherein
at least 50 percent to less than 100 percent of said sulfonic acid groups
are neutralized with a metal ion as described above; in other
embodiments, at least 59 percent may be neutralized; and from about 70
percent to about 90 percent neutralized in yet other embodiments.
Sulfonic acid groups not neutralized with the metal ion may be
neutralized with alkali or alkaline earth metal ions of Group 1 or 2 of
the Periodic Table, ammonium ions, or mixtures thereof.

[0049]The particulate catalyst may be employed by enclosing them in a
porous container such as cloth, screen wire, or polymeric mesh. The
material used to make the container may be inert to the reactants and
conditions in the reaction system. Particles of about 0.1 5 mm size or
powders up to about 1/4 inch diameter may be disposed in the containers.
The container used to hold the catalyst particles may have any
configuration, such as pockets, or the container may be a single
cylinder, sphere, doughnut, cube, tube, or the like.

[0050]Spacing component intimately associated with the catalyst component
may be provided to space the various catalyst components away from one
another. Thus, the spacing component provides in effect a matrix of
substantially open space in which the catalyst components are randomly
but substantially evenly distributed. One such structure is that shown in
U.S. Pat. No. 5,730,843, incorporated by reference herein. In addition,
commonly assigned U.S. Pat. Nos. 4,443,559, 5,057,468, 5,262,012,
5,266,546, and 5,348,710 disclose a variety of catalyst structures for
this use and are incorporated by reference herein.

[0051]U.S. Pat. No. 6,740,783, incorporated by reference herein, discloses
other catalysts useful for the production of dialkyl ethers from
alcohols, including crude alcohols containing some water. Disclosed are
hydrophobic zeolites serving as a catalyst, such as USY, mordenite,
ZSM-type, and Beta zeolites whose hydrogen cations are partially replaced
with suitable metal ions, such as Group 1, 2, 11, or 12 metal ions, or
ammonium ions. Other useful catalysts for the dehydration reaction are
disclosed in U.S. Pat. No. 3,931,349.

[0052]Catalysts used in the fixed bed reactor in various embodiments
disclosed herein may include metal-treated zeolites, either acidic or
basic, hydrofluoric acid-treated clays, and silica-alumina catalysts,
such as a 20% silica-alumina, among the other catalysts described above.
Catalysts used in the distillation column reaction zone may include
metalized resins and silica-alumina catalysts, among the other catalysts
described above. Metalized resin catalysts may include such catalysts as
zinc-treated AMBERLYST 15 and copper-treated AMBERLYST 35, among others.

[0053]In certain embodiments, the catalyst in the fixed bed reactor and
the catalytic distillation column reactor may include at least one of
H-ZSM-5, H-beta, H--Y, alumina, silica/alumina, macroporous cation
exchange resin with or without metals exchange, and combinations thereof.

[0054]The temperature profile across the distillation column reaction zone
should be sufficient to satisfy the kinetics of the alcohol dehydration
reaction. The temperature profile is also preferably sufficient to obtain
substantially complete conversion of the methanol. For example, for a
catalyst having high activity, temperatures and pressures may be less
severe than for a catalyst having a lower activity, where conditions for
each may be selected to satisfy the kinetics of the dehydration reaction
and to obtain substantially complete conversion of the methanol.

[0055]The severity of operating conditions in the pre-reactor may also
depend upon the amount of alcohol conversion required. The amount of
alcohol conversion required may also affect the choice of catalyst used
in the pre-reactor. For example, a desired pre-reactor conversion of 20
weight percent may require less severe operating conditions and/or a
lower activity catalyst than for a pre-reactor conversion approaching
equilibrium, 80 to 87 weight percent conversion.

[0056]The choice of catalyst and the severity of operating conditions in
the distillation column reaction system may also be affected by the
amount of alcohol conversion required. For example, the catalyst choice
and conditions may be different for a pre-reactor conversion of about 20
weight percent as compared to a pre-reactor conversion approaching
equilibrium.

[0057]Accordingly, catalysts used in the distillation column reactor
system may be the same or different than that used in the pre-reactor,
when present. In some embodiments, it may be preferred to use lower
activity catalysts in the distillation column reactor system, thus
allowing for extended catalyst life. Catalysts used in the pre-reactor
may be of a higher activity, such as where pre-reactors are run in
parallel, allowing for one to be repacked or regenerated while the other
is operational.

[0058]Distillation column operating conditions may also depend upon the
activity of the catalyst. For example, the amount of methanol converted
to dimethyl ether per distillation reaction stage may vary from 5 weight
percent to 50 weight percent or more. Distillation column operating
conditions, such as temperatures, pressures, and reflux ratios may need
to be adjusted to obtain substantially complete conversion of the
methanol. In some embodiments, reflux ratios may vary from about 0.1 or
0.5 to about 10; from about 0.5 to about 5 in other embodiments; from 0.6
to 3 in other embodiments; from 0.7 to 2.5 in other embodiments; and from
0.9 to 2 in yet other embodiments. In relation to alcohol conversion per
distillation reaction stage, higher reflux ratios are required at lower
conversion per stage. For example, for a methanol conversion per stage of
approximately 20 weight percent, the reflux ratio may range from 2 to 3
to obtain complete conversion of the alcohol, such as a reflux ratio of
about 2.4 in some embodiments. Comparatively, for a methanol conversion
per stage of approximately 40 weight percent, the reflux ratio may range
from 0.5 to 2 to obtain complete conversion of the alcohol, such as a
reflux ratio ranging from 1 to 1.6 in some embodiments.

EXAMPLE

[0059]The following example is derived from modeling techniques. Although
the work has been performed, these examples are not presented in the past
tense to comply with applicable rules.

[0060]A feed gas is processed in a system similar to that illustrated in
FIG. 2. The effluent from the methanol reactor has a composition as shown
in Table 1.

[0061]The methanol reactor effluent is fed to a methanol absorber
operating at a pressure of about 1200 psig, similar to the outlet
pressure of the methanol reactor, a temperature of about 375° F.,
and a gas to water mole ratio of about 5. Methanol is thus absorbed into
the water phase and transported to a catalytic distillation reactor
system for conversion of the methanol to dimethyl ether. The catalytic
distillation reactor system operates at an overhead temperature of about
150° F., a pressure of about 300 psig, and a reflux ratio (L/D) of
about 4, resulting in a bottoms fraction comprising water and having a
methanol content of about 1000 mole ppm.

[0062]The overheads from the catalytic distillation reactor system,
including dimethyl ether and light hydrocarbons, are fed to a
fractionation column operating at a pressure of about 300 psig, an
overhead temperature of about 122° F., and a relux ratio (L/D) of
about 1. The resulting dimethyl ether product, recovered as a bottoms
fraction, has a carbon dioxide content of about 1000 mole ppm.

[0063]Embodiments disclosed herein may provide for the effective
conversion of methanol to dimethyl ethers. Advantageously, various
embodiments may provide for one or more of substantially complete
conversion of the alcohol, recovery of an essentially pure ether
fraction, and recovery of an essentially pure water fraction.

[0065]Significantly, embodiments disclosed herein may provide for the
recycle of unreacted gases to the methanol synthesis reactor with a very
low compression and reheating requirement, including gases recovered
during methanol separations and following dimethyl ether production.
Advantageously, embodiments disclosed herein utilize an aqueous
extractant to recover methanol at high temperatures and high pressures,
where the aqueous extractant is easily recovered or consumed during
subsequent reaction of the methanol to form dimethyl ether, resulting in
the synergistically efficient separation and conversion of methanol from
a methanol synthesis reactor effluent.

[0066]While the disclosure includes a limited number of embodiments, those
skilled in the art, having benefit of this disclosure, will appreciate
that other embodiments may be devised which do not depart from the scope
of the present disclosure. Accordingly, the scope should be limited only
by the attached claims.